It is known to use a thin film of metal deposited on a flexible substrate such as a plastic sheet by vacuum electrodeposition for the purpose of heating foods in a microwave oven. Heaters of this kind which are known as susceptors provide a more intense heating effect at the surface of the food. The film of metal is thin enough to be electrically semiconductive so that during the heating process an electric current induced into the metal film from the electromagnetic field of the microwave oven produces I.sup.2 R losses which heat the food. The heating of food products by means of semiconductive vacuum electrodeposited metal films is exemplified by U.S. Pat. Nos. 4,230,924; 4,268,420; 4,258,086; 4,735,513; 4,641,005 and 4,678,882, and European patent application 0 205 304. In order to produce patches, i.e. rectangular metallized areas, the parts of the metallized film surrounding the patch are removed, i.e. totally demetallized, for example by the application of a caustic solution to the area that is to be removed. The dissolved metal is then washed off.
The demetallization of a metallized film is described for example in European application 0 205 304 and U.S. Pat. Nos. 3,647,508; 4,398,994; 4,522,614; and 4,735,513. The metal film is removed either by applying a caustic solution directly to the metal film or by covering portions of the metal film with a protective varnish and thereafter exposing the entire surface to caustic which dissolves the metal exposed beyond the edges of the varnish layer.
In the method described in U.S. Pat. No. 4,258,086, metal is removed by minute currents which pass between electrically conductive metal foil squares held adjacent to the coated film that is being treated. Using these methods, Beall and Brastad prepared demetallized films that have visible rectangular metallized patches or islands as small as 1/32nd inch on a side. These sheets are entirely covered with uniformly spaced visible rectangles. As a result, the heat produced by the sheet in a microwave oven is uniform throughout the entire sheet.
It is a primary object of the present invention to provide an improved method of partially demetallizing metal films so as to provide a metal film with gradations in optical density. Another object is to provide a semiconductive metallized film which is capable of producing differential heating, i.e. different amounts of heat in different areas thereof when exposed to microwave energy in a microwave oven. Yet another object is to provide a metallized sheet which is partially demetallized and wherein the degree of demetallization can be precisely controlled to thereby vary the optical density of the coating from one portion thereof to another for decorative or heating applications. A further object is to provide a demetallized metal film of the type described wherein the partially demetallized portions appear uniform, homogeneous and uninterrupted to the naked eye. Another object is to provide a unique microwave susceptor having a heating patch or target adapted to provide "focused" heating so as to produce a higher temperature near the center and a lower temperature at the periphery. Still another object is to provide a partially demetallized, semiconductive metal susceptor for microwave heating which is economical to produce, practical to manufacture, wherein the heat produced in different areas can be precisely controlled, and the various areas producing different amounts of heat can be given any desired shape.